Nowadays, exhaust-gas turbochargers are used as standard in order to increase the power of an internal combustion engine. The exhaust-gas turbocharger has an exhaust-gas turbine in the exhaust-gas tract of the internal combustion engine, and has a compressor which supplies air for the combustion process to the combustion chamber of the internal combustion engine. With the boosting of the internal combustion engine, the amount of air and fuel into the cylinders is increased, resulting in a noticeable increase in the power of the internal combustion engine. The exhaust-gas turbocharger which is used for this purpose comprises a rotor, having a shaft, a compressor wheel, a turbine wheel and if need be further components arranged on the shaft, as well as the shaft bearing, the flow-guiding housing parts (compressor housing, turbine housing) and the bearing housing.
As a result of the high process pressures, which are dependent on the current operating point of the exhaust-gas turbocharger, in the turbine-side and compressor-side flow area of an exhaust-gas turbocharger, the internal area of the bearing housing must be sealed from the flow channel by means of a shaft seal, in order to minimize gas leakage into the internal area. The pressure in the internal area normally corresponds to atmospheric pressure.
DE2025125 discloses a sealing apparatus for exhaust-gas turbochargers, in which one or two sealing elements is or are arranged on the compressor side, and at least one sealing element is arranged on the turbine side. The sealing elements are split piston rings which are pulled on over the turbocharger shaft and are inserted into grooves in the turbocharger shaft. The piston rings are adequately prestressed radially such that, when they are in the installed state, they rest radially on the stationary housing wall according to the laid-open specification, and remain positioned axially in the housing by virtue of the friction forces which occur in this case.
EP1130220 and EP1507106 disclose sealing apparatuses for exhaust-gas turbochargers, in which an axial stop shoulder is additionally provided on the stationary housing, on which stop shoulder the piston ring rests in an interlocking manner when a pressure difference is applied, thus remaining in a predetermined axial position. In this case, the piston ring is ground in on the rotating grinding-in surface, and thus seals the flow channel from the internal area of the bearing housing. The rotating piston ring groove is in this case slightly broader than the stationary piston ring. The pressure difference ensures that the piston ring is moved in the direction of the grinding-in surface, and is ground off in the process, until it rests on the axial stop shoulder on the housing hole. Grinding-in of the piston rings results in a better sealing effect, since this makes it possible to minimize the axial sealing gap between the piston ring and the grinding-in surface on the opposing piece. In general, seals without ground-in piston rings have higher leakage rates.
For tribological reasons, the grinding-in surfaces of the rotating opposing piece of a piston ring formed using gray cast iron (GG25) should not be composed of aluminum. The grinding-in surfaces on the rotating element should therefore always be formed from steel for conventional sealing apparatuses for exhaust-gas turbochargers with ground-in piston rings.
In the sealing apparatus for exhaust-gas turbochargers according to U.S. Pat. No. 5,176,497, the piston ring is secured axially by a screwed compressor wheel composed of aluminum and a sealing disk which also rotates on the turbocharger shaft. In this case, because of the pressure difference between the flow channel and the internal area of the bearing housing, the soft piston ring grinds itself in on the axial grinding-in surface of the sealing disk and thus produces the desired sealing effect. The compressor wheel is braced via the sealing disk by means of the screw connection to the turbocharger shaft. As disclosed in U.S. Pat. No. 4,986,733, the piston ring can also be mounted in a circumferential groove in the sealing disk, such that the piston ring does not make direct contact with the compressor wheel.
Building on this concept, in the case of the sealing apparatus for exhaust-gas turbochargers according to DE3737932, two piston rings are used as sealing elements for the turbine-side seal.
In the case of sealing apparatuses having two piston rings, the piston rings generally have different external ring diameters, for assembly reasons. In this case, the external diameter of the inner piston ring is less than the external diameter of the outer piston ring. Piston rings formed in this way allow the turbocharger shaft to be inserted axially, with the piston rings fitted, into the tapering housing opening. If the inner piston ring is provided with a larger external diameter than the outer piston ring, it is generally no longer possible to remove the inner piston ring without destroying the piston ring. In order to offer a hard grinding-in surface for the outer piston ring in the axial direction, an additional ring composed of a hard and wear-resistant material (for example steel) is required for conventional sealing apparatuses, and this is shrunk onto the compressor wheel between the inner and the outer piston ring. The extent of shrinkage requires very tight manufacturing tolerances. The process of shrinking on the steel ring can significantly influence the contact pattern of the thread and the loads in the screw connection between the compressor wheel and the shaft.